Graduate Thesis Or Dissertation

Numerical model investigations of wind-driven coastal circulation

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  • The effects of wind forcing on coastal ocean circulation are studied using a numerical modeling approach. The first region of interest is on the North Carolina shelf, where the Coastal Ocean Processes (CoOP) Inner Shelf Study (ISS) took place during August - November 1994. ISS observations are used to initialize, force, and compare results from a two-dimensional version (variations across-shore and with depth; uniformity alongshore) of the primitive equation Princeton Ocean Model. Both strongly stratified and weakly stratified conditions, found during August and October, respectively, are studied. An additional difference between these two periods is that August is characterized by fluctuating alongshelf winds, while October is dominated by downwelling-favorable winds. Momentum term balances, across-shelf transport values, and Lagrangian dynamics are contrasted for the August and October periods. The nonlinear advection terms contribute significantly to the alongshelf momentum balance in depths less than 10 m during upwelling, but not during downwelling. This result regarding the asymmetry in the shelf response during upwelling and downwelling motivates further study in this region using the same model setup with forcing by periodic alongshelf wind stress. This periodicity allows further investigation of the upwelling-downwelling asymmetries and the Lagrangian characteristics of the flow. An important result of the asymmetric upwelling and downwelling responses is a non-zero mean Eulerian and Lagrangian velocity over the forcing period. These mean velocities differ, leading to a mean Stokes velocity that is largest in the complex region near the coast where parcel trajectories are irregular. A Lagrangian mapping technique and calculation of the largest Lyapunov exponent help identify the nature of fluid parcel displacements over many periods. Focus of the modeling effort then shifts to the region of the northern California shelf, where the CoOP Wind Events and Shelf Transport (WEST) program collected measurements during January 2000 May 2003. The three-dimensional Regional Ocean Modeling System (ROMS) primitive equation model is initialized and forced with WEST observations, and extensive model-data comparisons are made with three WEST datasets. The model compares reasonably well with moored velocity measurements, CODAR surface current observations, and shipboard hydrographic measurements. The mean response over the summer period is dominated by an upwelling circulation, including a coastal jet that separates off Pt. Arena and Pt. Reyes. Flow near Pt. Reyes during an upwelling and relaxation wind event is complicated and the response north and south of the cape is quite different. Lagrangian results reveal that the source of upwelled water near the coast south of Pt. Reyes is local, as compared to that north of Pt. Reyes, which has a signature of deeper water from farther north. Momentum term balances help to clarify the event dynamics as a function of shelf location. The Lagrangian analyses include both a parcel tracking and label advection technique and provide detailed information on the upwelling response of fluid parcels on the shelf.
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